The long-term goal of this proposal is to understand how circadian clocks function within eukaryotic cells. The purpose of a circadian clock is to regulate cellular processes such that they occur at specific times of the day and night. Circadian clocks are found in all kingdoms of life and the presence of a functional circadian clock has been shown to confer enhanced fitness onto the organism. Forward genetic approaches to understanding clock function have been instrumental in numerous model organisms including Arabidopsis and our initial gene discovery program yielded a key clock gene, TOC1, and the founding member of a novel photoreceptor family, ZTL. The success of this gene discovery program validates the approach, although currently circadian screens in Arabidopsis are not saturated since we are still identifying novel clock genes. We will continue the characterization of existing mutants and isolate novel mutants by developing new reporters based on TOC1, a critical component identified from our previous screens. In addition, we will exploit reverse genetic approaches to explore hypotheses about the role of clock gene family members in the circadian clock. Given the ubiquity of circadian-regulated physiology, the identification of common clock components will have an impact on understanding the pacemaker mechanism and malfunctions associated with known features of human well-being. [unreadable] [unreadable]